Syrian special forces have massacred scores of men in a sweep through the Syrian city of Hama in what is believed to have been the bloodiest retribution so far in President Hafez Assad's two-year crackdown on opponents to his rule, according to an eyewitness account and diplomatic reports in Washington, Europe and the Middle East.

In the sweep through several Hama neighborhoods in late April, adult and teen-age males were jerked from their homes in the middle of the night, lined up against walls and machine-gunned with bullets large enough to have torn up their bodies, a witness reported. Reports on the number killed vary, with the most reliable estimating between 150 and "several hundred." According to a Hama resident who later fled, municipal garbage trucks picked up bodies from the streets and police buried them in holes scopped out by ditch-diggers.

Reprts of the attack have circulated in the Middle East for several weeks and The Washington Post has delayed publication until it could independently confirm through diplomatic sources in Washington that such a raid did take place.

The assault on a city long regarded as a center of anti-Assic agitation demonstrates the level of concern in Assad's government over the persistent resistance to his authority and his determination to crush it with ruthless use of force if necessary. It was designed, the reports say, as punishment for several terrorist raids in the preceding few days against militiamen from Assad's Arab Baath Socialist Party and, according to some accounts, the machine-gunning of participants ina spring festival in fields near a village inhabited by members of Assad's minority Alawite Moslem sect.

The attacks and the government's harsh revenge both seem to cast doubt on recent assessments that Assad has largely succeeded in his campaign to eliminate sedition and opposition among Syria's 70 percent Sunni Moslem majority and, in particular, in reactionary Moslem Brotherhood cells said to be spearheading antigovernment terrorism.

The terror attacks and the retribution happened between April 22 and April 28, according to conflicting diplomatic reports in Washington and Paris. In a version published May 13 by the Paris newspaper Le Monde, quoting a Syrian Moslem Brotherhood leader involved in the anti-Assad agitation, the sweep on Hama occurred April 25. According to a former Hama resident who says he was there at the time it was in the early hours of April 24.

The massacre reports, in trustworthy and untrustworthy variations, have been discussed in Damascus and Beirut in the last two months. In an atmosphere created by the wounding last June of Reuter correspondent Bernd Debusmann, shot in the back by a gunman firing a silencer-equipped pistol, and threats against British Broadcasting Corp. correspondent Tim Llewellyn -- both after stories considered by Damascus as unfriendly to Syria -- the Hama report have not been widely published from the area.

Hama, in a rich agricultural region between Homs and Aleppo in central Syria, traditionally has been a center of Sunni fundamentalism and, according to the government in Damascus, the outlawed Moslem Brotherhood. The brotherhood, a Sunni movement, has taken the lead in organizing opposition to Assad, protesting that he has stacked key government and Army posts with members of his 12 percent Alawite minority and objecting to his Baath Party's secular philosophy.

It was the massacre of about 50 mostly Alawite Army cadets in June 1979 that began the open war on Assad's government. Since then, diplomats in Damascus estimate that roughly 500 Batthists and other government supporters have been murdered and up to 2,000 antigovernment agitators have been killed. Hama was singled out for specially harsh retribution late last year and early this year, with up to 200 residents killed in a series of indiscriminate shellings to end what was described as a near insurrection.

Col. Rifaat Assad, the president's brother and commander of the "Protection Brigades" that operate as a palace guard, was reported to have commanded the operations. The Protection Brigades also carried out last April's massacre, along with Syria's "Special Units" commanded by Gen. Ali Haidar, an Alawite and a trusted Assad aide, according to the reports.

According to the former Hama resident, the sweep began with a helicopter landing about 1 a.m. April 24. Soldiers burst into apartments at random and took out teen-age boys and men, he said. There was no resistance, he added, because people thought it was a raid such as those carried out earlier in which the men arrested would be questioned about antigovernment activities and released.

This time, however, the soldiers had different orders, he said, and gunfire rattled through the neighborhood until 9 a.m. In a taped interview recalling what happened, granted on condition that he remain anonymous, he went on:

"About 11 o'clock, I recognized the voice of my brother in the street. He does not live with us because he is married. I heard him scream, "They didn't leave a man, not a boy, in the neighborhood.'

"I came down, putting on my shirt. I walked a few steps before coming on a pile of bodies, then another. There must have been 10 to 15. I walked by them, one after the other. I looked at them a long time, without believing my eyes. . . . In each pile, there were 15 bodies, 25, 30 bodies. The faces were totally unrecognizable.

"I think the bursts were shot at head level because I saw bits of brains on the ground and one the walls. And then, when I saw the bullet holes, it couldn't have been anything but 50-caliber machine guns, not Kalashnikovs (AK47 assault rifles, the standard Syrian Army issue). I don't think a Kalashnikov could tear up bodies like that. Mush. There were bodies of all ages, 14 and up, in pyjamas, gelebiyehs , in sandals or barefoot."